This invention relates to the scrubbing of noxious substances from exhaust gas streams, particularly those found in the semiconductor industry.
Large quantities of noxious substances are employed in the semiconductor industry in the processing of electronic wafers/chips and other devices. In the case of plasma enhanced chemical vapour deposition (PECVD) in particular, process gases including silane, TEOS, phosphine and dichlorosilane are used in turn between the different process stages in the same process chamber held under vacuum by vacuum pumps.
Between the stages, cleaning steps are required for the process tools and these are commonly effected using perfluorocarbon (PFC) gases. The cleaning steps tend to use a relatively small proportion of the PFC gases and the unused proportion is discharged to the exhaust duct from the process chamber.
Both the process gases and the cleaning gases are evacuated from the process chamber in turn via the same exhaust system and generally need to be scrubbed from the exhaust gas stream so that it is rendered harmless before passing to the atmosphere. A variety of different types of scrubbing equipment has been attached to exhaust systems for this purpose.
It is normal practice in the semiconductor industry to add considerable amounts of nitrogen gas to the exhaust stream being passed through the vacuum pumps to assist the pumping operation itself and to dilute any pyrophoric gases present in the exhaust stream to below their threshold limit of combustion. In addition it is generally deemed expedient in the semiconductor industry that the unused PFC gases are destroyed or recovered but the dilution with nitrogen renders it more expensive and difficult to treat or recover the unused PFC gases in the exhaust stream.
There is therefore a need to provide an improved method for the efficient scrubbing of such exhaust gas streams coupled with an ability to treat or recover unused PFC gases.